19. Gradients and Ecotones

Question 1

What causes environmental gradients? 2

Answer 1

1. physical and chemical properties of water

2. interaction with surrounding environment

Question 2

what important nutrients my be limiting in freshwaters? 4

Answer 2

1. Nitrogen
2. Phosphorous
3. Silicon
4. These influence the biology and chemistry of the freshwater system

Question 3

Describe the nitrogen cycle in freshwater. 10

Answer 3

1. Most nitrogen fixed in soil
2. Precipitation of bioavailable N only important in oligotrophic (low nutrient) lakes
3. Nitrogen fixation does occur in water
4. by cyanobacteria (blue-green algae)
5. occurs largely in heterocysts
6. generally low but significant in eutrophic lake
7. Inorganic nitrogen from
   groundwater and surface run-off largely NO3- and NH4+, but amount of NH4+ influenced by pH:
   NH4+ ↔ NH4OH
8. At higher pH, less NH4+
9. Inorganic nitrogen is the main form
10. ammonium (nh4+) is important in plant biodiversity

Question 4

Describe the phosphorous cycle in freshwater. 7

Answer 4

1. Phosphate (PO4) main source of P to plants
2. Largely via erosion of land but tightly bound and so not bioavailable – e.g. Apatite
   Ca10(PO4)6(OH,F,Cl,Br)2 - must be broken down
3. P also adsorbed onto clays and ferric hydroxide
4. Both deposited in the sediments therefore major reservoir for P
5. Not easily released - non-bioavailable
6. Solubility (and hence availability) dependent on conditions in sediments – increased by:
   
   • low dissolved oxygen
   • low pH
7. Availability comes from soil and run off

Question 5

How does the presence or absence of oxygen influence the phosphorous cycle? 4

Answer 5

1. in anoxic conditions, phsophorous is in form of phosphate
2. in oxic conditions, iron is insoluble iron hydroxide thar binds phosphate so low bioavailability
3. Diffusion of oxygen low in sediment (max 1-2cm) so release of P inhibited: marked PO4 gradient at sediment/water interface
4. Mixing is important to prevent eutrophication in artificial lakes, as disturbance of anoxic sediment releases phosphorous

Question 6

What factors influence the phosphorous cycle in water? 8

Answer 6

1. FALL IN PH
2. solubilise apatitie, and so releases PO4
3. de-absorbs PO4 due to replacement with H+
4. SEDIMENT DISTURBANCE
5. anaerobic bacteria – CH3 and H2S
6. bioturbation (e.g. chironomid larvae); feeding by fish
7. MACROPHYTES
8. roots oxygenate sediment and reduce phospherous

Question 7

What are the characteristics and physical constraints of freshwater systems? 6

Answer 7

1. Wetland - land area saturated with water, either permanently or seasonally
2. Rivers & streams – running or lotic waters (from L. lavere – to wash)
3. Lake/pond - static fresh or salt water in a basin surrounded by land.
4. Standing or lentic waters (L. lentus - sluggish)
5. Stratification less likely in shallow ponds and lakes (> 5m)
6. No agreement on definition of pond:
   
   1. light regime: “standing body of water where light penetrates to the bottom”
   2. size: but no agreement - max area varies from 5 to 8 hectares

Question 8

Describe deep freshwater lakes - deeper than 5m. 4

Answer 8

1. Photic zone: part of water column - where light penetrates
2. Distinction between shoreline (littoral) and central zone: macrophytes in photic zone
3. In summer, stratified water column differentiated into autotrophic epilimnion and heterotrophic hypolimnion
4. Benthic community: macrophytes and algal mats in photic zone; heterotrophic (bacterial-dominated) in aphotic zone

Question 9

Describe shallow, unstratified lakes and wetlands. 5

Answer 9

1. Photic zone: whole of water column
2. Distinction between littoral and central zone: macrophytes in both zones
3. Depths

Question 10

Describe an erosional river/stream system. 5

Answer 10

1. Photic zone: throughout water column
   Distinction between littoral and central zone: little difference
2. Allochthonous input > autochthonous
3. Allochthonous – imported material, e.g. deciduous leaves; autochthonous – produced within the system, e.g. phytoplankton, benthic algae, macrophytes
   4.Benthic community: epibenthic algae as high flow prevents rooted macrophytes and phytoplankton
4. Sediment: course so oxic and hence aerobic heterotrophic community

Question 11

Describe a depositional river/stream. 6

Answer 11

1. Photic zone: throughout water column
2. Distinction between littoral (shore) and central zone: pronounced, with emergent macrophytes in littoral zone
3. Allochthonous input =/ few mm

Question 12

Describe the availability of organic matter in the 4 freshwater ecosystems. 4

Answer 12

1. Primary production within the system (autochthonous) forms majority of organic matter in standing waters
2. Less contribution from littoral zone in upland nutrient-poor standing waters
3. Imported organic matter (autochthonous) contributes most in erosional streams and rivers
4. Some contribution to autochthonous by plankton in very slow flowing rivers

Question 13

What is an ecotone? 6

Answer 13

1. Zone of transition, and interaction, between adjacent biological systems (from Greek tonos or tension)
2. Important in freshwaters
3. wide: e.g. wetlands – transition from aquatic to terrestrial
4. narrow: stream with bed-rock channel
5. often higher biological diversity and density – the edge effect
6. edge species – those that use edges for reproduction and survival

Question 14

What is the ecotone edge affect, and how does this manfiest itself at River Sinnamary? 5

Answer 14

1. Higher biological diversity and density – the edge effect
2. River Sinnamary, French Guiana - tropical watercourse with number of types of vegetation
   3 Edges had highest number of fish species: 46 compared to 24 in channel
3. due to allochthonous (imported from outside) sources (e.g. leaves, fruit, flowers)
4. important to fish populations